1. Field of the Invention
This invention relates generally to color video cameras, and more particularly is directed to circuits for automatically controlling characteristics of the camera, for example, for exercising automatic iris, white balance and/or black balance controls.
2. Description of the Prior Art
In general, the white portion of a picture obtained with a color video camera is maintained constant by means of a white balance correction or control even if there is a change in the color temperature of the light source illuminating the object in the field of view of the camera. Although it is well known to effect a manual correction or control of the white balance while a sheet of white paper or the like is held in the field of view of the camera, more expensive or technically sophisticated color video cameras usually provide automatic correction of the white balance. In the existing automatic white balance correction circuits, the brightest portion of an object or picture in the field of view is presumed to be white and is detected to generate a corresponding gating pulse during which the amplitudes of the red, green and blue color signals being generated by the camera are made equal to each other. More specifically, the gating pulse is usually generated by equally adding the red, green and blue color signals obtained from the image device of a color video camera, and comparing the resulting added signal with a so-called semi-peak voltage which is generated by the use of a diode clamp circuit effecting a predetermined reduction from the peak voltage level of the added color signal. However, since objects normally in the field of view of the camera do not usually include substantial pure white portions, or the white portions may have yellow areas included therein, the gating pulse may be generated, not only in response to truly white portions, but also in response to comparable bright portions of the object in the field of view. Therefore, precise white balance correction may not be realized.
In known automatic iris control circuits, a peak detector having a diode and a capacitor is employed for obtaining exposure data, that is, data indicating the amount of light incident on the image pick-up device of the video camera. However, the characteristic of such peak detector depends on the characteristics of the diode and capacitor so that the operating mode of the automatic iris control is variable only within a relatively narrow range, that is, the lighting conditions under which the video camera can be used are narrowly limited.
Although it is known to also effect black balance control of a color video camera, the provision of a color video camera with aromatic iris, white balance and black balance controls requires a complex circuit arrangement which is difficult to incorporate in an integrated circuit.